The present invention relates to a Doppler radar apparatus for observing the Doppler velocity and intensity of meteorological echoes by use of an FMICW modulated signal with a high pulse repetition frequency.
FIG. 13 is a diagram showing a system of related-art Doppler radar apparatus, for example, disclosed in the Unexamined Japanese Patent Application Publication No. 2000-275329. In FIG. 13, the system includes: a first highly stable local oscillator 1 for generating a signal at a frequency f1-fif; a second highly stable local oscillator 2 for generating a signal at a frequency f2-fif; a switching circuit 3 for switching the signals from the first and second highly stable local oscillators 1 and 2 alternately every pulse; a mixer 4 for mixing an output signal from the switching circuit with a signal at a frequency fif; an IF local oscillator 5 for generating a signal at the frequency fif; a high pass filter 6 for passing only signals at frequencies f1 and f2; a pin modulator 7 for pulse-modulating a signal; a transmitting tube 8; and a polarized-wave switching circuit 9 for switching a transmission path in accordance with a polarized wave.
The system further includes circulators 10 and 11; a mixer 12 for mixing the signals from the local oscillators 1 and 2; a low pass filter 13 for extracting a signal at a frequency fclk; a control circuit 14; TR limiters 15 and 27 for protecting a reception circuit from transmitted waves leaking into the reception circuit; high-frequency amplifiers 16 and 28; mixers 17 and 29; polarized component extracting filters 18 and 30 each for extracting a reception signal at the frequency fif; 90-degree shifters 19 and 31 each for providing a phase difference of 90 degrees; mixers 20, 21, 32 and 33; filters 23, 24, 34 and 35 each for extracting a Doppler signal; and A/D converters 25, 26, 36 and 37 each for converting an analog signal into a digital signal.
In meteorological radars for observing rainfall or snowfall, the velocity of the wind of an echo (raindrop or snowflake) can be gauged by use of the Doppler effect. When the wind is measured with a Doppler radar, however, the number of pulses shot per second (Pulse Repetition Frequency (PRF)) cannot be increased sufficiently in relation between the maximum observable range Rmax and the maximum observable velocity Vmax so that the measurement of the Doppler velocity is limited by phenomena as follows. One is a problem that folding of the Doppler velocity is generated, and the other is a limit in the observable range for suppressing the generation of secondary echoes.
C-band meteorological radars usually observe intensity in an observable range up to about 250 km. The PRF is limited by this maximum range Rmax of 250 km in the following relation.
PRFxe2x89xa6C/(2xc3x97Rmax) (C represents light velocity)xe2x80x83xe2x80x83(1) 
Therefore, the PRF cannot be made higher than 3xc3x97108 m/s/(2xc3x97250 km)=600 Hz. On the other hand, in Doppler observation, the maximum velocity Vmax is limited in the following relational expression.
(xcex designates wavelength, which is about 5.6 cm when the frequency is in the C-band ranging from 5,250 MHz to 5,350 MHz)
Vmaxxe2x89xa6|xcex/2xc3x97PRF/2|xe2x80x83xe2x80x83(2) 
Normal meteorological radars need to be not lower than 40 m/s as their observable range of Doppler velocity. When the PRF is about 600 Hz, the observable range is, however, limited to 8.4 m/s in accordance with the expression (2). Accordingly, when Doppler observation is carried out, the PRF is increased to about 1,000 Hz to obtain the maximum velocity Vmax of 14 m/s, while observation is carried out a plurality of times with different PRFs. Thus, xe2x80x9cprocessing of folding of Doppler velocityxe2x80x9d is carried out to secure xc2x140 m/s or higher while the observable range is set to about 150 km.
(Detailed Description of Folding of Doppler Velocity)
In radars, a reception signal is obtained in every period corresponding to the PRF. The phase shift between the discrete signals received thus is used for the work of estimating an original continuous wave after measuring some points of the continuous wave. Therefore, as shown by a sampling theorem, the measuring limit of Doppler velocity fd is expressed by PRF/2 so that folding of Doppler velocity occurs in a frequency higher than PRF/2. Doppler velocity shown in the following expression (3) is called xe2x80x9cfolded velocity (Nyquist velocity) Vnyq. Actually, the wind velocity higher than Vnyq really exists as described above. Thus, outputted (folded) Doppler velocity Vr in the case of velocity V0 higher than Vnyq is expressed by:
Vr=V0xc2x1nxc3x97Vnyq (n=2, 4, 6 . . . )xe2x80x83xe2x80x83(3) 
FIG. 14 shows a phenomenon of velocity folding. Since normal meteorological radars use approximately xcex=5.6 cm, PRF=896 Hz, and Vnyq=12.5 m/s, for example, actual Doppler velocity obtained when the wind velocity is 20 m/s is 20xe2x88x92(2xc3x9712.5)=xe2x88x925.0 (m/s). Accordingly, in actual Doppler radars, correction processing for canceling folding by use of two kinds of frequencies as PRF is carried out to expand the wind space measurable range into about three times as wide as Vnyq.
(Detailed Description of Secondary Echo)
For such a reason, in Doppler radars, the PRF is set to be relatively high to obtain Vnyq as high as possible. As a result, the Doppler radars cannot help making the Doppler observable range Rmax narrower than the intensity observable range in accordance with the expression (1). The Doppler observable range Rmax becomes 167 km at the PRF of 896 Hz. In this case, there appears a false target (called a secondary echo) if an intensive echo exists in a range exceeding the radius of 167 km which is the observable range. It has been therefore necessary to take separate measures to remove the secondary echo.
Next, description will be made on the operation of the related-art apparatus. In the related-art apparatus, two kinds of frequencies are used to double the observable range of Doppler velocity. The operation at that time will be described below.
Signals at frequencies f1-fif and f2-fif are outputted from the first and second local oscillators 1 and 2 respectively. These signals are switched alternately in a transmission repetition period (the reciprocal of the repetition frequency PRF) by the switching circuit 3, and transmitted with their polarized waves changed, respectively. The repetition period with which two reception systems corresponding to those polarized waves receive the signals is twice as long as the transmission repetition period (that is, the repetition frequency is half as high as the transmission repetition frequency). Accordingly, the maximum velocity Vmax of each received signal becomes halved in each of the two reception systems in accordance with the expression (2). However, since the signals are transmitted and received alternately, Doppler velocity can be measured at the initial repetition frequency by use of both the received signals.
For example, assume that the repetition frequency is 1,000 Hz. In this apparatus, signals are transmitted at the frequencies f1 and f2 with their polarized waves changed. Thus, the signals are independent of each other, and the repetition frequencies of their own become half, that is, 500 Hz, respectively. Accordingly, the maximum range Rmax observable in each of the respective reception systems is 300 km. On the other hand, Doppler velocity can be also observed up to velocity equivalent to 500 Hzxc3x972=1,000 Hz by continuous processing of signals received alternately. Thus, the observable range of Doppler velocity is doubled to have the relation Rmax=C/PRF. Incidentally, since the periods of alternate transmission and reception are overlapped, the polarized waves of signals for transmission and reception are changed to prevent the signals from interfering with each other in this apparatus.
In the related-art Doppler radar apparatus, there has been a problem that the scale of the apparatus is so large that the cost increases because two kinds of polarized waves (horizontal one: H and vertical one: V) are switched and transmitted alternately in such a manner as described above. In addition, as for the frequency band, the frequencies f1 and f2 have to be prepared. Thus, there has been a problem that the apparatus is apt to interfere with other radar apparatus in terms of radio license. On the other hand, even with such a configuration, the pulse repetition frequency cannot be increased over about 1,000 pps on a large scale. Therefore, xe2x80x9ccorrection processing of velocity foldingxe2x80x9d using two kinds of pulse repetition frequencies has been required as described in the related art.
The invention is achieved to solve the foregoing problems. It is an object of the invention to provide a Doppler radar apparatus which can measure velocity up to the maximum velocity of xc2x140 m/s or higher by use of a single pulse repetition frequency without performing xe2x80x9ccorrection processing of velocity foldingxe2x80x9d. It is another object of the invention to provide apparatus in which, in addition to the previous object, the observable range of Doppler velocity can be expanded to be as wide as the intensity observable distance range. Further, it is another object of the invention to provide a method for realizing such radar apparatus.
A Doppler radar apparatus according to the invention include: a first oscillator for generating a first sweep signal to repeatedly sweep a predetermined frequency range periodically; a second oscillator for generating a second sweep signal having sweep properties identical to those of the first sweep signal, the second oscillator starting sweep before the first oscillator finishes frequency sweep; a synthesizer for synthesizing the first and second sweep signals to generate a transmission signal; a switch for receiving the first and second sweep signals as inputs, and switching an output between the first and second sweep signals synchronously with timing when sweep with each of the first and second sweep signals is terminated; and a mixer for mixing a reception signal reflected in a target, and an output signal from the switch with each other to produce an output signal from the mixer.
Further, the Doppler radar apparatus according to the invention may include an A/D converting unit for converting the output signal from the mixer into a digital signal, and a first range FFT processing unit for applying FFT processing to the digital signal to compute distance information.
Further, the Doppler radar apparatus according to the invention may include a filtering unit for weighting an amplitude component of the signal subjected to the distance FFT processing so that frequency characteristic is in inverse proportion to an amplitude value corresponding to clutter to thereby remove the clutter component from the digital signal.
Further, the Doppler radar apparatus according to the invention may include a weighting unit for weighting frequency characteristic before the A/D conversion so that loss is large in a lower frequency and minimal in a higher frequency.
Further, the Doppler radar apparatus according to the invention may include first and second amplifiers for amplifying the first and second sweep signals respectively, so that the synthesizer synthesizes signals amplified by the first and second amplifiers.
Further, the Doppler radar apparatus according to the invention may include a second FFT processing unit having a number of process points different from that in the first range FFT processing unit, and provided at an output of the A/D converting unit in parallel with the first FFT processing unit.